System for setting coarse GPS time in a mobile station within an asynchronous wireless network

ABSTRACT

A method and apparatus for setting coarse GPS time in a GPS receiver in a mobile station (MS) that is communicating with a base station and a position determining entity (PDE). The MS requests an assistance message from the PDE that includes a sequence of predicted navigation bits, including a predicted time indicator field, which is then located and decoded. Coarse time is set responsive to the time indicator value. A Pattern Match Algorithm may be performed to provide more precise GPS time. In order to better set coarse time, an expected error in the Time of Week may be determined, by for example using the expected network latency. The system describe herein enables the use of IS-801 protocol by an MS in asynchronous networks by improving the coarse time setting process.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.60/489,652, filed on Jul. 23, 2003.

FIELD OF THE INVENTION

The present invention generally relates to position location systemsthat determine the position of a mobile station, such as a cellularphone, by use of wireless signals.

DESCRIPTION OF RELATED ART

Existing position location techniques based on global positioning system(GPS) satellites utilize a network of satellites, commonly known asspace vehicles (SV's), that transmit signals that are accurately phasereferenced to GPS time. A GPS receiver on the ground measures therelative times of arrival of the signals from each “in view” SV (i.e.,each SV from which the receiver can receive signals). The relative timesof arrival of the signals along with the exact location of the SVs areused to determine the position of the GPS receiver using a techniquecommonly known as trilateration. A relatively accurate estimate of GPStime at the time the signals were transmitted from each SV is requiredin order to accurately determine the location of each SV at the time thesignals were transmitted. For example, the SV's motion relative to earthcan be as much as 950 meters/sec. The location of the SV is calculatedusing a mathematical equation that predicts the location of an SV in itsorbit at a particular point in time. Due to the velocity of the SV, asingle millisecond of time error would equate to an SV position error ofup to 0.95 meters. The resulting error in the calculated position of theGPS receiver may vary. However, a general rule of thumb is that onemillisecond of time error will result in an error of about 0.5 meters inthe calculated position of the GPS receiver.

In order to know the exact time that the signals were transmitted fromthe SVs, a standard GPS receiver either demodulates the time oftransmission from the received signal or maintains a clock bias estimatethat estimates the difference between the local receiver clock and GPStime. Establishing the time bias between the GPS receiver's free runningclock and GPS time is often referred to as “setting the clock”. If theSV signal is received by the GPS receiver in good condition, then theGPS receiver can set the clock based on information contained in thereceived signal. The information received indicates the time oftransmission. However, even in the best of conditions, setting the clockmay consume considerable time (e.g. up to six seconds or more) due tothe amount of time required to receive the necessary informationtransmitted by the SV. Furthermore, in environments in which the signalis blocked or otherwise weakened, the GPS receiver can never set theclock to GPS time, and therefore can never determine its position.

Another way to set the clock is to synchronize the clock with areference clock that has a known relationship to GPS time. For example,synchronizing to GPS time is straightforward in a CDMA mobile station(MS) (such as a cellular phone) used in a CDMA network. This is becauseCDMA networks are synchronized to GPS time. Being synchronized to GPStime means that the transmissions from each of the base stations withinthe network are referenced to GPS time. Accordingly, the CDMA receiverin the MS has knowledge of GPS time. The operating software within theMS can simply transfer this GPS time to the GPS receiver software by,for example, relating the GPS time to a precise hardware signal or pulsewhich allows the GPS receiver software to associate the GPS time withits own clock time in a precise fashion. As discussed above, priorknowledge of precise GPS time inside the GPS receiver can significantlyshorten the time needed to determine the location of a GPS receiver(commonly referred to as “obtaining a GPS fix”). Particularly in noisyenvironments, prior knowledge of precise GPS time may become important,or even essential in obtaining a GPS fix.

For quicker and more efficient determination of GPS fixes in CDMAsystems, the Electronics Industry Association/TelecommunicationsIndustry Association (EIA/TIA) adopted a standard known as the “IS-801standard”, or simply “IS-801”. IS-801 includes a set of rules (commonlyreferred to as “protocols”). The protocols prescribe the data contentand sequence of messages that can be exchanged between a positionlocation server (commonly referred to as a PDE) and an MS. These IS-801messages help the GPS receiver measure pseudoranges and/or generatelocation fixes. For example, IS-801 messages include requests for“ephemeris”. Ephemeris is information regarding the orbits of the SVs.IS-801 messages also include other aiding information, such asinformation regarding the bit patterns that the SVs are expected tosend. Predicting the bits allows the GPS receiver to perform coherentintegration over longer periods of time. This in turn increases thesensitivity of the GPS receiver.

However, some cellular networks, such as the Global System for MobileCommunication (GSM) networks, are not synchronized with GPS time. Suchsystems are referred to as “asynchronous”. Accordingly, the GPS receiverin an asynchronous network does not have direct access to GPS time fromthe communication signal. In the presence of noise or if the signalsfrom the SVs are attenuated, a GPS system that does not have the luxuryof attaining GPS time from the communication system may take longer todetermine a GPS fix. In the extreme case, if there is too much noise,determining a GPS fix may become impossible. One method for determiningGPS time in an asynchronous system is referred to as a “Pattern Match”method. In a Pattern Match method, the time at which GPS signals arereceived at the MS is compared with the time at which GPS signals arereceived at a reference receiver that is synchronized to GPS time.Assuming that the distance between the transmitting SV and the referencereceiver is essentially equal to the distance between the transmittingSV and the GPS receiver, the time at which the signals are received bythe reference receiver can be used to set the clock in the GPS receiver.However, since the information that is transmitted by the GPS SVs isrepeated, effective operation of the Pattern Match method requires thatthe MS be “coarsely” synchronized with GPS time, for example to within afew seconds. Otherwise, it is impossible to tell whether the informationreceived by the GPS receiver was transmitted at the same time as theinformation received by the reference receiver.

For example, assume that the same information is transmitted by aparticular GPS SV every two seconds. Further assume that it is possiblefor the clock within the GPS receiver to be offset by as much as twoseconds from the clock within the reference receiver. Now assume thatboth the clock within the reference receiver and the clock within theGPS receiver indicated that the information in question was received atexactly 12:00PM. Since we don't know what time the information wasreally received by the GPS receiver, it is possible that the informationwas actually received at 12:00PM, two seconds before 12:00PM, or twoseconds after 12:00PM. That is, the information received by the GPSreceiver might be information that was actually sent by the SV at thesame time as the information received by the reference receiver, twoseconds earlier, or two seconds later. Accordingly, there is no way oftelling whether the clocks in the reference receiver and the MS areperfectly synchronized or out of synchronization by two seconds.

The coarse time synchronization ensures that the clock within the MS issynchronized to GPS time with sufficient accuracy to ensure that thepattern match method can determine the exact time without ambiguity.Several methods are known for establishing coarse time synchronization.In one method, a transmit and acknowledge pair of messages are used. Forexample, the MS transmits a request for time and simultaneously starts alocal timer. The BTS receives the request from the MS and acknowledgesreceipt of the request by sending the current time. The MS receives thetime estimate from the BTS. The MS then stops the local timer and readsthe elapsed time. Such systems can assist in establishing coarsesynchronization, but add cost, can become complicated, and can introduceundesirable time delays. Accordingly, there is a need for a faster andmore efficient system for setting coarse GPS time in a GPS receiver.

SUMMARY OF THE INVENTION

The method and system described herein enables the use of IS-801protocol intended for use only in synchronous networks to be used by amobile station (MS) in an asynchronous network by improving the processused to set coarse time. One implementation of the disclosed method andsystem allows a “Pattern Match” algorithm to more precisely set thereceiver's clock to precise GPS time.

A method is described herein for setting coarse GPS time in a GPSreceiver of a mobile station (MS) that is communicating with a positiondetermining entity (PDE) through a base station. The GPS receiver isconfigured to periodically receive transmitted navigation bits from aplurality of SVs synchronized with GPS time. The navigation bits includeat least one time indicator field. The MS requests a sensitivityassistance (SA) message from the PDE. The message includes a sequence ofpredicted navigation bits. In responsive to the request from the MS, theSA message is sent from the base station approximately in time with GPStime. The SA message is received in the MS and the time of receipt issaved. A predicted time indicator field is located within the predictednavigation bits. In response to the located time indicator field, apredicted “Time of Week” (TOW) is determined. Responsive to thepredicted TOW, coarse GPS time is set within the GPS receiver to reflectthat the predicted navigation bits were received at the time indicatedby the predicted TOW. Using the coarse time, the GPS receiver can fixthe location of the GPS receiver more quickly and efficiently. Forexample, responsive to the coarse GPS time and the predicted navigationbits, a Pattern Match Algorithm may be performed to provide precise GPStime.

In order to better set coarse time, an expected error in the TOW may bedetermined by using the expected network latency. Then, the step ofsetting coarse GPS time within the GPS receiver may include adjustingthe time to take into account the expected error due to the networklatency.

The method disclosed herein enables use of the conventional IS-801messages to assist in fixing the location of a GPS receiver inasynchronous networks such as GSM or UMTS (Universal Mobile TelephoneService). In a described embodiment, the transmitted navigation bitshave a format that includes a plurality of frames. Each frame isorganized into a plurality of subframes. Each subframe has a “timeindicator” field, such as a “Time of Week” field. The SA message in theIS-801 standard includes at least one subframe of predicted navigationbits. In such embodiments, the method may further comprise locating a“predicted time indicator” field within a subframe of the predictednavigation bits, and calculating the TOW in response to the predictedtime indicator.

In some embodiments, the SA message includes a “data length” field thatspecifies the length of the predicted navigation bits, and a “ReferenceBit Number”. The Reference Bit Number locates an “Actual Reference Bit”within a frame of the actual navigation bits with respect to the firstbit of the frame that includes the Actual Reference Bit.

The particular bit selected as the Actual Reference Bit is selectedbecause it corresponds to a Predicted Reference Bit that is at a knownlocation within the stream of predicted navigation bits. The location ofthe Predicted Reference Bit is know with respect to the beginning of thestream of predicted navigation bits. By locating the Actual ReferenceBit with respect to the beginning of the frame and the PredictedReference Bit with respect to the beginning of the stream of predictednavigation bits, each of the fields within the entire stream ofpredicted navigation bits can be identified and located.

Once located, the time indicator field within the predicted navigationbits is decoded to provide a “predicted time indicator”. Responsive tothe predicted time indicator, a TOW at which the Predicted first bit ofthe sequence of predicted navigation bits is estimated to have beenreceived is determined. Accordingly, coarse GPS time is set at the timethe Predicted first bit of the sequence of predicted navigation bits wasreceived within the GPS receiver based upon the TOW at which thePredicted first bit of the sequence of predicted navigation bits isestimated to have been received. The predicted time indicator is definedwith regard to a weekly time reference. The step of determining the TOWmay comprise computing a “Bit of Week” corresponding to the number ofbits elapsed from the weekly time reference until the first bit of thesequence of predicted navigation bits. The step of computing the Bit ofWeek may include determining if the Predicted first bit of the sequenceof predicted navigation bits is in the same subframe as the predictedtime indicator, and responsive thereto, adjusting the predicted timeindicator.

Additionally, methods are disclosed to adjust computation of the TOW totake into consideration boundary conditions such as week rollover (wherethe subframe in which the TOW is positioned immediately precedes thetransition at the end/beginning of a week), and the case where the firstbit of the sequence of predicted navigation bits and the TOW field liein different, adjacent frames.

The method can be implemented in an MS for determining positionutilizing periodically transmitted navigation bits from a plurality ofSVs synchronized with GPS time. The periodically transmitted navigationbits include a time indicator field. The MS also communicates with oneor more base stations and a position determining entity (PDE).

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this invention, reference is nowmade to the following detailed description of the embodiments asillustrated in the accompanying drawing, wherein:

FIG. 1 shows a plurality of cellular base stations, GPS SVs, and a userholding a mobile device such as a cell phone;

FIG. 2 is a block diagram of the mobile device in one embodiment thatincorporates communication and position location systems;

FIG. 3 is a diagram of the message structure in a GPS signal, includingframes, subframes, and words;

FIG. 4 is a diagram of the structure of the GPS Sensitivity Assistance(SA) message 41 as prescribed by the IS-801 protocol;

FIG. 5 is a flowchart of steps to set coarse time from using a SAmessage in asynchronous networks; and

FIG. 6 is a bit mapping showing the correspondence between the predictednavigation bits, a frame of a GPS message, and the TOW field.

DETAILED DESCRIPTION

In the following description, reference is made to the Figures, in whichlike numbers represent the same or similar elements.

Glossary of Terms and Acronyms

The following terms and acronyms are used throughout the detaileddescription:

GPS: Global Positioning System. Although the term GPS is often used torefer to the U.S. Global Positioning System, the meaning of this termincludes other global positioning systems, such as the Russian GlonassSystem and the planned European Galileo System.

CDMA: Code Division Multiple Access. CDMA is a high-capacity digitalwireless technology that was pioneered and commercially developed byQUALCOMM™ Incorporated. CDMA is a major commercial rival to the GSMstandard.

GSM: Global System for Mobile Communications. GSM is a widely usedalternative digital wireless technology.

UMTS: Universal Mobile Telephone Service. UMTS is a next generation highcapacity digital wireless technology.

MS: Mobile Station. MS is any mobile wireless communication device, suchas a cell phone that has a baseband modem for communicating with one ormore base stations. MS's referenced in this disclosure include a GPSreceiver to provide position determination capabilities.

BS: Base Station. A BS is an entity that communicates with a mobilestation, for example a BS may include a BTS, a Mobile Switching Center(MSC), Mobile Positioning Center (MPC), a PDE, and any InterworkingFunction (IWF) useful for network connections.

BTS: Base Transceiver Station. A BTS is fixed station used forcommunicating with mobile stations. Includes antennas for transmittingand receiving wireless communication signals.

SV: Space Vehicle. A set of SVs compose one major element of the GlobalPositioning System. The SVs orbit the Earth and broadcast uniquelyidentifiable signals among other information.

Pseudorange measurement: A pseudorange measurement is a measurement madefor the purpose of determining the relative distance between atransmitter and receiver. A process employed by GPS receivers and basedon signal processing techniques to determine the distance estimatebetween the receiver and a selected SV. The distance is measured interms of signal transmission time from the SV to the receiver. “Pseudo”refers to the fact that the clock of the SV and the receiver are notsynchronized. Therefore, the measurement contains an uncompensated clockerror term.

PDE: Position Determination Entity. A PDE is a system resource (e.g. aserver) typically within the CDMA network, working in conjunction withone or more GPS reference receivers, which is capable of exchanging GPSrelated information with an MS. In an MS-Assisted A-GPS session, the PDEsends GPS assistance data to the MS to enhance the signal acquisitionprocess. The MS returns pseudorange measurements back to the PDE, whichis then capable of computing the position of the MS. Alternatively, inan MS-Based A-GPS session, the MS sends back computed position resultsto the PDE.

GPS SA message: Global Position Sensitivity Assistance Message. The GPSSA message is defined in the IS-801 protocol. The GPS SA messageincludes predicted navigation bits of the currently visible SVs. Thenavigation bits are predicted by the PDE and sent from the PDE to the MSin OTA (“over-the-air”) format.

IS-95: IS-95 refers to the industry standard document published by theTelecommunications Industry Association/Electronics IndustryAssociation, TIA/EIA-95-B, entitled “Mobile Station—Base StationCompatibility Standard for Wideband Spread Spectrum Systems”.

IS-801: IS-801 refers to the industry standard document published by theTelecommunications Industry Association/Electronics IndustryAssociation, TIA/EIA/IS-801, entitled “Position Determination ServiceStandard for Dual-Mode Spread Spectrum Systems—an adjunct standard toIS-95 and IS-2000-5 that describes the protocol between an MS and a PDE.

GPS fix: A GPS fix is the end result of a process of measurements andsubsequent computations by which the location of the GPS user isdetermined.

IS-801 session: An IS-801 session is the sequence of data exchangebetween an MS and a PDE in a manner prescribed by the IS-801 standardfor the purpose of getting a position fix. The sequence typicallycontains various GPS assistance data messages sent by the PDE andpseudorange or position result sent by the MS. Session beginning ismarked by the time when either side initiates a data exchange sequenceby a request and the session ends when the initiating side terminatesthe exchange sequence with a session-ending message.

Mobile-Terminated (MT) session: An MT session is an IS-801 sessioninitiated by the PDE.

Mobile-Originated (MO) session: An MO session is an IS-801 sessioninitiated by the MS.

OTA (“over-the air”) format: OTA format is the format in which a messageis physically transmitted.

Overview

The system described herein provides a method for “coarsely settinglocal time” in a mobile station (MS). The system includes receiving a“Timing Assist” message that includes the same information (andformatting) as a portion of a navigation message that is concurrentlybeing transmitted by another source, such as a GPS satellite. Includedwithin the Timing Assist message is a Reference Bit. The time of arrivalof the Reference Bit is noted by the local clock within the MS. Assumingthat the Timing Assist message was transmitted at a time properlycalculated to cause the Timing Assist message to arrive at the MS at thesame time as the navigation message transmitted from the GPS SV, therelative time at which the Reference Bit was received can be calculatedfrom information (e.g., a predicted time indicator field) containedwithin the Timing Assist message, as will be explained in more detailbelow.

On advantage of the presently disclosed method and apparatus is that itextends the use of the IS-801 protocol, originally designed forsynchronous networks, to asynchronous networks in order to assist indetermining the location of a mobile station (MS). In general, by firstutilizing a method described herein to set the coarse time, a PatternMatch algorithm can be used to improve on the coarse time and set thereceiver's clock to precise GPS time. The method of setting coarse timedescribed herein is useful because without the presently describedmethod, the IS-801 messages exchanged between the MS and the server insynchronous systems are generally not useable in a system that utilizesan asynchronous network. Particularly, in the IS-801 standard, there isno dedicated message defined for the purpose of time transfer. Thisdisclosure describes a method for inferring an estimate of time fromIS-801 messages.

FIG. 1 illustrates one environment in which the coarse time settingsystem described herein can be implemented. In one describedenvironment, a GPS receiver and cell phone are implemented together in amobile station (MS) 14. However, it should be apparent that the presentinvention could be used with any other type of mobile station (otherthan cell phones) that communicate with one or more land-based stations.Furthermore, the MS and GPS receiver need not be integrated together,but may instead be electrically coupled either by direct connection orwireless communication.

FIG. 1 shows a plurality of cellular base stations collectively referredto by reference numeral 10, GPS satellites, commonly referred to asspace vehicles (SVs), collectively referred to by reference numeral 11,and a user 13 grasping an MS 14. As described in more detail withreference to FIG. 2, the MS 14 includes a position location system 27,such as a GPS system, and a communication system 22, such as a cellphone, that utilizes two-way communication signals 20 to communicatewith the cellular base stations 10. The user 13 may be on foot as shown,or may be traveling in a car or on public transportation, for example.For ease of description, the position location system 27 is referred toherein as a “GPS” system; however, it should be recognized that thesystem described herein could be implemented in any one of several othertypes of positioning systems.

The SV's 11 comprise any group of SVs utilized for positioning a GPSreceiver. The SVs are synchronized to send out wireless signals 12synchronized to GPS time. These signals are generated at a predeterminedfrequency, and in a predetermined format described in more detailelsewhere herein. In a current GPS implementation, each SV transmits aGPS signal on the L-frequency band (at which the GPS receiver operates).As discussed in the background, when the GPS signals 12 are detected bythe GPS receiver 29 in the MS 14, the GPS system 27 attempts tocalculate the relative amount of time elapsed from transmission of theGPS signal 12 until reception. In other words, the GPS system 27calculates the difference in the amount of time required for each of theGPS signals 12 to travel from their respective SVs 11 to the GPSreceiver 29. The relative measurement is referred to as a pseudo range.The pseudo range is defined as: c•(T_(user)+T_(bias)−T_(sv)), where c isthe speed of the GPS signal 12, T_(user) is the GPS time when the signal12 from a given SV 11 is received, T_(bias) is the difference betweenthe time according to the user clock and the actual GPS time, and T_(sv)is the GPS time when the SV 11 transmitted the signal 12. In the generalcase, the receiver 29 needs to resolve four unknowns: X, Y, Z (the EarthCentered Earth Fixed coordinates of the receiver antenna), and T_(bias)(the offset between the Receiver's GPS time estimate and the true GPStime when the signals 12 are received). For this general case, resolvingthe four unknowns usually requires measurements from four different SVs11. However, under certain circumstances, this constraint can berelaxed. For example, if an altitude estimate is available, then thenumber of SVs 11 required can be reduced from four to three, since thealtitude measurement can be used to define the value in the Z direction,leaving only three unknowns to be resolved.

The cellular base stations 10 comprise any collection of cellular basestations utilized as part of a communication network that communicateswith the MS 14 using wireless signals 20. The cellular base stations 10are connected to a cellular infrastructure network 15 that providescommunication services with a plurality of other communication networkssuch as a public phone system 16, computer networks 17 such as theInternet, a position determination entity (PDE) 18 (defined above), anda variety of other communication systems shown collectively in block 24.A GPS reference receiver 19, which may be in or near the base stations10 or any other suitable location, communicates with the PDE 18 toprovide useful information in determining position, such as a GPS clock.

The ground-based cellular infrastructure network 15 typically providescommunication services that allow the user 13 of a cell phone to connectto another phone using the phone system 16; however the cellular basestations could also be utilized to communicate with other devices and/orfor other communication purposes, such as an internet connection with ahandheld personal digital assistant (PDA). In one embodiment, thecellular base stations 10 are part of a GSM communication network;however in other embodiments other types of asynchronous communicationnetworks may be used.

FIG. 2 is a block diagram of one embodiment of the mobile device 14incorporating communication and position location systems.

A cellular communication system 22 is shown in FIG. 2 connected to anantenna 21 that communicates using the cellular signals 20. The cellularcommunication system 22 comprises suitable devices, such as a modem 23,hardware, and software for communicating with and/or detecting signals20 from cellular base stations, and processing transmitted or receivedinformation.

A position location system 27 in the MS, in this embodiment a GPSsystem, is connected to a GPS antenna 28 to receive GPS signals 12 thatare transmitted at or near the ideal GPS frequency. The GPS system 27comprises a GPS receiver 29, a GPS clock 30 (that may allow for clockbias and an uncertainty factor), and any suitable hardware and softwarefor receiving and processing GPS signals and for performing anycalculations necessary to determine position using any suitable positionlocation algorithm. Some examples of GPS systems are disclosed in U.S.Pat. Nos. 5,841,396, 6,002,363, and 6,421,002, by Norman F. Krasner. TheGPS clock 30 is intended to maintain accurate GPS time, however sinceaccurate time is frequently unknown, it is common practice to maintaintime in the GPS clock software by its value and an uncertaintyassociated with that value. It may be noted that after an accurate GPSlocation fix, the GPS time will be very accurately known, (to within afew nanoseconds of uncertainty in current GPS implementations).

A mobile device control system 25 is connected to both the two-waycommunication system 22 and the position location system 27. The mobiledevice control system 25 includes any appropriate structure, such as amicroprocessor, memory, other hardware, firmware, and software toprovide appropriate control functions for the systems to which it isconnected. It should be apparent that the processing steps describedherein are implemented in any suitable manner using one or morehardware, software, and/or firmware components, subject to control bythe microprocessor.

The control system 25 is also connected to a user interface 26, whichincludes any suitable components to interface with the user, such as akeypad, a microphone/speaker for voice communication services, and adisplay such as a backlit LCD display. The mobile device control system25 and user interface 26, connected to the position location system 27,provide suitable functions for the GPS receiver, and the two-waycommunication system, such as controlling user input and displayingresults.

FIG. 3 is a diagram of the standard message structure in a GPS signal20. In one implementation, the SV transmits a sequence of frames at arate of fifty bits per second (50 bps). The message structure includes a1500-bit long frame 31 made up of five subframes 32. Each subframecontains ten words 34, each word 34 being thirty bits long. Of thesethirty bits, six bits are designated as parity bits; the remaining 24bits are source data bits. These 24 source data bits are the so-called“navigation” bits.

In one current GPS implementation, before transmission, each SV convertsthe 24 navigation bits in each word to over-the-air (OTA) format bymodulo-2, adding to each of them the last computed parity bit of theprevious word (the so-called D30 bit 35). Accordingly, if the D30 bit 35is a logical “1”, then each of the source data bits are inverted. If theD30 bit 35 is a logical “0”, then the source data bits are leftunaffected. Then, the remaining six parity bits of the word are computedusing a Hamming Code. When the SV signal is received at the GPSreceiver, the message is decoded from its OTA format in order toretrieve the source data bits. As discussed below in more detail, theacquisition of the SV message can take some time because it proceeds atthe relatively slow pace of 50 bps. In noisy environments accuratedecoding may be difficult (or impossible) to achieve. Furthermore, thetime information (what is referred to herein as a “SUBFRAME COUNT” Fieldin the current GPS implementation) only occurs every six seconds, whichmeans that the opportunities to decode the time sequence are fairlyinfrequent. In a noisy environment decoding can be problematic, and oneor more opportunities to determine the time sequence may be missed,which can cause lengthy time delays before the time information can besuccessfully decoded.

Each 300-bit subframe 32 begins with a 30-bit “telemetry” (TLM) word 36.The TLM word 36 is followed by a 30-bit handover word (HOW) 37. The HOWincludes the 17 most significant bits of a 19-bit value. That 19-bitvalue is sometimes referred to as the “time of week” (TOW). The 17-bitlong field comprising the 17 most significant bits of the TOW is what isreferred to herein as the SUB-FRAME COUNT. The SUB-FRAME COUNT is apredicted time indicator that is used in the currently disclosed methodfor coarse time setting. Particularly, the value in the SUB-FRAME COUNTfield indicates the time at the start of the following subframe relativeto the beginning of the week. Because the SUB-FRAME COUNT is reset tozero at the beginning of each week and increments every 6 seconds, theSUB-FRAME COUNT can be used as a subframe counter.

For the purposes of this document, a “subframe epoch” is the instant intime when one subframe period stops and the next one starts. TheSUB-FRAME COUNT is limited to a range from 0 to 100,799. It should benoted that 100,800 times 6 seconds is equal to the number of seconds ina week. It should be clear that 100,800 is also the number of sub-framestransmitted each week. At the end of each week (i.e., when the SUB-FRAMECOUNT reaches the maximum value), the SUB-FRAME COUNT is reset to zero.Accordingly, the first state of the SUB-FRAME COUNT (i.e., the SUB-FRAMECOUNT value of zero) occurs at the subframe epoch which is coincidentwith the start of the present week. (In the current GPS implementation,this epoch occurs at midnight Saturday night—Sunday morning, wheremidnight is defined as 0000 hours on the Universal Time Coordinated(UTC) scale, which is nominally referenced to the Greenwich Meridian.)

It should be noted here that the SUB-FRAME COUNT is sometimesconfusingly referred to as either the “Handover Word” (HOW) or “Time ofWeek” (TOW). However, this will not be the case in the present document.

If the GPS receiver can receive SV signals in good condition from theGPS SVs, then the receiver can demodulate the navigation bitstransmitted by the visible SVs and therefore the receiver would becapable of decoding the SUB-FRAME COUNT. The SUB-FRAME COUNT can then beused to set a clock within the receiver to GPS time. However, receivingthe SUB-FRAME COUNT may take up to six seconds because the SUB-FRAMECOUNT occurs only once in every subframe, i.e., once every six seconds.Furthermore, in environments in which the signal is blocked or otherwiseweakened, data bit demodulation is not always possible, or if possiblemay consume considerable time. Therefore, in order to enable rapidcoarse time setting in asynchronous networks regardless of signalconditions, a system is described that utilizes a particular message(the GPS Sensitivity Assistance message) of the IS-801 protocol.

The Sensitivity Assistance (SA) message is supplied from the PDE 18 overthe cellular communication signal 20. The SA message can be processed asdescribed herein to provide a predicted HOW (rather than the actualHOW), which can then be used to set coarse time.

Reference is now made to FIG. 4, which is a diagram of the structure ofthe SA message 41 as prescribed by IS-801. The intended purpose of theSA message 41 when proposed was to provide sensitivity assistance to theMS when determining position. The SA message 41 is one type ofassistance message, and it should be clear that a different format forthe assistance message might be used. However, for convenience ofdescription, the fields are referenced herein by their IS-801 names.

In the current implementation, the SA message 41 can include up to eightparts 42. Each part 42 can contain up to sixteen data records. Eachrecord is uniquely associated with one SV. Each data record can includea Predicted Navigation Bits field 46 and a Satellite PRN Number field 47(SV_PRN_NUM). The Predicted Navigation Bits field 46 can contain up to510 predicted navigation bits. The Predicted Navigation Bits field 46 ofthe currently visible SVs is sent by the PDE 18 (FIG. 1) to the MS 14 inOTA format. Accordingly, the encoding of the predicted navigation bitsfollows the same algorithm as the OTA encoding used by the SVs as notedabove. Because of this OTA encoding, the receiver must decode theSUB-FRAME COUNT field in the predicted navigation bits received in theSA message from the PDE 18 in order to use the SUB-FRAME COUNTinformation to set coarse GPS time.

In addition to the records and associated fields within the records,each part 42 of the SA message includes a number of additional fields.Some of these additional fields are illustrated in FIG. 4 with theircurrent IS-801 designation following in parentheses. A “Reference BitNumber” field 43 (REF_BIT_NUM) conveys the position of an “ActualReference Bit” within the 1500-bit GPS frame sent by the SVs. The ActualReference Bit is that bit within the actual navigation bits sent fromthe SV that is associated with the last bit of the first half of thestream of predicted navigation bits in the SA message (hereafterreferred to as the “Predicted Reference Bit”). Further informationregarding the use of the Reference Bit will be provided below.

A “Data Record Size” field 44 (DR_SIZE) specifies the length of eachdata record that includes the predicted navigation bits. In the currentimplementation, the value of DR_SIZE is indicated in 2-bit increments.

A “Number of Data Records” field 45 (NUM_DR_P) specifies the number ofdata records in the part. In one implementation, each data record isassociated with a single SV, therefore the Number of Data Records field45 also designates the number of SVs for which information is providedin the part, up to 16.

The PDE 18 is capable of predicting the values of navigation bitsemitted by the SVs at a time that is not to distant in the future basedon the fact that many fields of the navigation bits are constant.Furthermore, those bits that are not constant change from their currentstate in a mostly predictable manner. The reference receiver 19communicates to the PDE 18 the value of the navigation bits beingtransmitted by the SVs. Accordingly, the PDE 18 knows the values of thenavigation bits most recently transmitted by the GPS SVs. The PDE 18uses the values of the navigation bits received from the referencereceiver 19 to predict the values of the navigation bits that will betransmitted in the future. Particularly, the predicted navigation bitsare predicted by the PDE 18 based upon the knowledge that the values ofthe navigation bits repeat or knowledge that the values they representwill increment periodically by a known amount at a known rate over time.

In one example, in a synchronous CDMA network, the PDE 18 sends an SAmessage to the MS with 496 predicted navigation bits for each visibleSV, which is equivalent to 9.92 seconds worth of navigation bits. Insynchronous networks the predicted navigation bits in the SA message areused to increase the GPS receiver's sensitivity. However, as describedherein, in asynchronous networks the SA message is used for a completelydifferent purpose. That is, the SA message is used in an asynchronousnetwork to set coarse time. As long as the PDE 18 sends at least sixseconds worth of navigation bits for at least one SV, it will be certainthat a full HOW can be found somewhere in the predicted message. Thispredicted HOW can be decoded. From the decoded HOW, the GPS receiver'sclock can be set to a coarse GPS time.

FIG. 5 is a flowchart of steps to set coarse time from the IS-801 typemessage in asynchronous networks. FIG. 6 shows the correspondencebetween the predicted navigation bits, a frame of a GPS message, and theSUB-FRAME COUNT field. In the following discussion, reference is made tothe IS-801 standard for purposes of illustration. It should be apparentthat the method could apply to other position determination systems.However, the disclosed method and apparatus is most useful in anasynchronous system in which an IS-801 message source is available. TheMS 14 (FIG. 1) is in communication with the base stations, and receivesa request, such as from a user, to determine the position of the MS.

At 51, an IS-801 type session is initiated (for example either an MO orMT IS-801 type session), and during the IS-801 type session (preferablyat the beginning of the session), the MS requests an assistance message(SA data in IS-801 type format) from the PDE 18 (FIG. 1).

At 52, in response to the request from the MS, the PDE 18 predictsfuture navigation bits utilizing its GPS reference receiver 19, andforms the SA message shown in FIG. 4. When forming the SA message, thePDE 18 sets the Reference Bit Number field to indicate the positionwithin the 1500 bit SV message frame (in the range from 0 to 1499) ofthe Actual Reference Bit that corresponds with the Predicted ReferenceBit. The PDE 18 also sets a value for the Data Record Size field, whichspecifies the length of the Predicted Navigation Bits field. The PDE 18arranges for transmission of the SA message from a BTS at a time thatwill cause the first bit of the Predicted Navigation Bits field of theSA message to be received by the MS at approximately the same time asthe receiver in the MS would receive the corresponding bit of the actualnavigation bits from the GPS SVs.

At 53, upon arrival of the SA data from the PDE 18, the receiversoftware notes the time indicated by the local clock when the first bitof the SA message was received. In an alternative method, the time atwhich some other particular part of the SA message, such as thePredicted Reference Bit of the predicted navigation bits, is receivedmight be noted. The PDE 18 saves the indicated time. The receiver thendecodes the SA message in order to determine the content of the message.It should be noted that while the first bit of the Predicated NavigationBits field in the SA message is the reference point in time forsynchronizing the predicted navigation bits with the actual navigationbits, any other well-defined reference within the stream of actualnavigation bits could be used. However, the fact that the first bit ofthe Predicted Navigation Bits field in the SA message is easy to detectmakes it a convenient choice. It should also be noted that the rate atwhich the stream of predicted navigation bits is received by the MS istypically far greater than the rate at which actual navigation bits aretransmitted from the SVs. However, as long as the first bit of thePredicted Navigation Bits field of the SA message arrives at the MS atapproximately the same time as the corresponding bit transmitted by theSVs (or at a known offset in time), the presently described method willbe effective.

At 54, after decoding the SA message, the values of the Reference BitNumber field 43 (FIG. 4) and the Data Record Size 44 will be known tothe MS. Using this information, the predicted SUB-FRAME COUNT fieldwithin the predicted navigation bits is located as discussed in greaterdetail below with reference to FIG. 6.

Reference is now made to FIG. 6 in conjunction with FIG. 5. As discussedabove, the value of the Reference Bit Number field 43 conveys theposition of the Actual Reference Bit 61, (shown in FIG. 6), within the1500-bit GPS frame sent by the SVs. It should be clear that the locationof the Actual Reference Bit shown in FIG. 6 is only one example of thelocation of the Actual Reference Bit. In actual practice the ActualReference Bit may fall anywhere within the frame 31. The ActualReference Bit 61 of the SV message corresponds with a bit 62 in themiddle of the decoded predicted navigation bits (i.e., the PredictedReference Bit 62). Since the value of the Reference Bit Number field 43indicates the distance of the Actual Reference Bit 61 from the start ofthe frame 31 (i.e., a value from 0 to 1499, as noted above), thedistance to the nearest preceding SUB-FRAME COUNT field 66 within thestream of predicted navigation bits can be easily calculated.

In accordance with one implementation, the Predicted Reference Bit 62 isalways the last bit of the first half of the stream of predictednavigation bits in the SA message. Then, using the knowledge of thelength of the Predicted Navigation Bits field, the receiver software candetermine where the SUB-FRAME COUNT field 66 lies within the stream ofpredicted navigation bits. It should be noted that the location of theHOW that includes the SUB-FRAME COUNT field 66 always starts at locationbit 30, 330, 630, 930, and 1230 referenced from the start of the frame.This is because the format of the SV navigation message is rigid.Therefore, if the Reference Bit Number field has a value of 1201 and theData Record Size 44 has a value of 398, then the first bit of the SAPredicted Navigation Bits is the 1001, which is the location of theActual Reference Bit, 1201 (as provided by the Reference Bit Numberfield 43) minus one half of the length of the data record (398/2) plusone=200.

Accordingly, since there are 300 bits in each of the five words of the1500 bit frame, the first bit of the Predicted Navigation Bits fieldwould correspond to the 101^(st) bit of the fourth word. Clearly, theSUB-FRAME COUNT in the fourth word will not be included since theSUB-FRAME COUNT in the fourth word occurs in bits 31-60, but theSUB-FRAME COUNT of the fifth word that occurs 229 bits into the SAPredicted Navigation Bits field, would be provided.

Referring again to FIG. 5, at 55, the MS then decodes the locatedSUB-FRAME COUNT field 66. In the current implementation, decoding a wordwithin the stream of predicted navigation bits requires that the D30 bit35 of the preceding word be available (See FIG. 3). Therefore, in orderto decode the HOW word 37, the D30 bit 35 of the preceding (TLM) word 36must be available within the predicted navigation bits. Accordingly, thefirst decodable HOW word in the predicted navigation bits must bepreceded by a D30 bit. In the example provided above in which the firstbit of the Predicted Navigation Bits field is bit 1001, the D30 bitwould occur 198 bits into the Predicted Navigation Bits field(1199−1001). Accordingly, the D30 bit preceding the SUB-FRAME COUNT inthe fifth word would be available.

In one implementation the following substeps are performed to decode theSUB-FRAME COUNT:

1. Determine the position within the bit 64 of the SV navigation messageframe that corresponds to the first bit of the Predicted Navigation Bitsfield 63 within its subframe (i.e. position 0-299). In the exampleprovided above, that position is represented by the value 1001−900=101.

2. Determine the position of the beginning of the first decodable HOWword relative to the first bit of the Predicted Navigation Bits field 63and save the 17-bit SUB-FRAME COUNT value. In the example providedabove, that position is represented by the value 229. It should be notedthat there could be more than one complete HOW word within the stream ofpredicted navigation bits. For example, if there are 496 predictednavigation bits, then two complete HOW words (and therefore two completeSUB-FRAME COUNT fields) may be available. For convenience of descriptionit will be assumed that the first HOW word will be chosen for decoding.Alternatively, any other of the HOW words within the stream of predictednavigation bits could be decoded.

3. Determine the position of the D30 bit of the word directly precedingthe HOW (the TLM word in this implementation) relative to the first bitof the Predicted Navigation Bits field and save this D30 bit value. Inthe example provided above, that position would be represented by thevalue 198. In one implementation, in the event that the D30 bit of thedirectly preceding word is not provided with the predicted navigationbits, then this SUB-FRAME COUNT Field cannot be decoded, and in thatevent the next SUB-FRAME COUNT Field would be chosen for decoding.

4. Decode the SUB-FRAME COUNT: invert the bits of the SUB-FRAME COUNT ifthe D30 bit has a binary value of “1” to obtain the decoded value of theSUB-FRAME COUNT from its OTA value. If the D30 bit has a binary value of“0”, then the bits of the SUB-FRAME COUNT are ready for use.

At 56, the decoded SUB-FRAME COUNT value and the position of theSUB-FRAME COUNT Field within the predicted navigation bits are used todecode the SUB-FRAME COUNT value. It will be understood that theSUB-FRAME COUNT value refers to the beginning of the sub-frameimmediately following the sub-frame that includes the decoded SUB-FRAMECOUNT field 66 referenced from the beginning of the week. It may benoted that, in the IS-801 implementation, the length of the PredictedNavigation Bits field 46 is made long enough to include at least oneSUB-FRAME COUNT value, and possibly two. As noted above, the SUB-FRAMECOUNT value has a value in a range from 0 to 100,799 (100,800 possiblevalues) that represents the number of subframes that have occurred sincethe start of the week. Furthermore, as discussed above, a subframe istransmitted every six seconds. Accordingly, the SUB-FRAME COUNTindicates the number of six-second intervals since the beginning of theweek at midnight Saturday night—Sunday morning on the UniversalCoordinated Time scale, which is nominally referenced to the GreenwichMeridian.

A process for determining the time indicated by the SUB-FRAME COUNTfield 66 is now disclosed. It should be noted that the time indicated bythe SUB-FRAME COUNT is the time at the start of the sub-frame followingthe sub-frame that includes the SUB-FRAME COUNT. Noting that there are300 bits per sub-frame and each bit persists for 20 milliseconds, thetime with respect to the beginning of the week is calculated (e.g.time=300*SUB-FRAME COUNT*20 ms). This calculated time indicates thenumber of milliseconds elapsed from the beginning of the current GPSweek to the time the subframe associated with the SUB-FRAME COUNT wastransmitted by the SV. For the purpose of setting the clock to a coarsetime value, the difference between time of transmission and time ofreception can be neglected. It should be noted that in the exampleprovided above, the SUB-FRAME COUNT is from the fifth subframe.Accordingly, the time calculated from the SUB-FRAME COUNT is the time atwhich the next frame begins. That is, the time is 1501 minus 1001 bitsafter the first bit of the Predicted Navigation Bits field was received.

The time of the first bit within the Predicted Navigation Bits field ofthe SA message is determined. This process is described with referenceto 54 and to FIG. 6. In the example provided above, the position of thefirst bit of the Predicted Navigation Bits field has a value of 1001 andthe time is (300*SUB-FRAME COUNT*20 ms)−((1501−1001)*20 ms).Alternatively, the location of the first bit of the Predicted NavigationBits field with respect to the first bit of the subframe to which theSUB-FRAME COUNT relates could be first subtracted. That is, the time atthe first bit of the Predicted Navigation Bits field could be calculatedas ((300*SUB-FRAME COUNT)−(1501−1001))*20 ms. In yet another alternativeembodiment of the current method, the SUB-FRAME COUNT value can beadjusted to point to the beginning of the subframe that carries theSUB-FRAME COUNT.

After such an adjustment, position 1001 of the first bit of thePredicted Navigation Bits field would be subtracted from the position1201 of the first bit of the subframe that carries the SUB-FRAME COUNT.The time at the beginning of the Predicted Navigation Bits field wouldthen be calculated as (300*(SUB-FRAME COUNT−1))−(1201−1001)*20 ms.

It should be noted that the minimum length of the Predicated NavigationBits field is 330 bits in order to ensure that the needed D30 bit isavailable. Furthermore, subtracting from the SUB-FRAME COUNT has to betaken modulo 100,800 to avoid negative values if the predictednavigation bits span the week rollover.

In another example using the IS-801 standards, assume the PredictedNavigation Bits field has a length of 500 bits, and the Reference BitNumber field has a value that designates a position in the GPS frame atbit 700. Then, since the Reference Bit Number corresponds to the lastbit of the first half of the predicted navigation bits, the first bit ofthe Predicted Navigation Bits field is 700−500/2+1=451. So the first bitof the Predicted Navigation Bits field is the 451st bit of the frametransmitted by the SV. Each subframe has a 300-bit length, each word hasa 30-bit length, and the HOW word is the second word in each subframe.Therefore, bit #451 is located in the second subframe, after the HOWword. In other examples, it is possible that the first bit of thePredicted Navigation Bits field reaches back to the previous GPS frame,so the calculation of the location of the first bit of the PredictedNavigation Bits field from the Reference Bit Number must be done using amodulo 1500 subtraction (i.e., 1−2=1499).

Referring again to the flowchart of FIG. 5, at 57, the error(uncertainty or “coarseness”) in the coarse time is estimated. It shouldbe recognized that the coarseness (i.e., the amount of uncertainty) insetting the GPS time can be approximately bounded. This is because thecoarseness depends primarily on the transmission latency of the networkwithin which the PDE 18 sends the SA message to the MS. The transmissionlatency in turn depends on the mode of transmission employed within thegiven network. Accordingly, the latency can be measured and/orpredetermined. So, when the PDE 18 sends the SA message, the majority ofany time inaccuracy can be attributed to the time it takes to send thebits from the PDE 18 to the receiver, which is termed the “networklatency”. In one example, the network latency may be in the range ofseveral seconds, however, adjustments can be made to account for thatlatency.

Thus, this uncertainty typically takes into account network latency butmay take into account other factors; alternatively, this error may bepredetermined based upon expected network latency conditions. So, in thestep 52, when the PDE 18 sends the SA message approximately synchronizedwith the actual timing of the navigation message sent by a GPS SV, thenthe majority of any time inaccuracy can be attributed to the time ittakes to send the bits from the PDE 18 to the receiver, which is termedthe “network latency”, and may be in the range of several seconds.

At 58, the GPS receiver's clock 30 (FIG. 2) is set to the coarse time atthe time of receipt of the beginning of the Predicted Navigation Bitsfield in the SA message. The clock's bias is set to zero, and the timeuncertainty is set to the predetermined error value. As discussed above,the coarse time has an uncertainty in the accuracy, which primarily canbe attributed to the network latency. In other words, the value to whichthe clock is set is accurate only within the limits of this uncertaintyresulting from the network latency. Since accurate time is most oftennot known to a GPS receiver prior to fixing the location of thereceiver, it is common practice to maintain time in the GPS clocksoftware by its value and an uncertainty associated with that value. Inthis case the uncertainty of the coarse time may be in the range ofseveral seconds.

At 59, the predicted navigation bits are passed to a method, such as aPattern Match Algorithm, which then determines precise GPS time. In oneembodiment, the predicted navigation bits are passed in unchanged, OTAformat from the SA message to the Pattern Match algorithm, which is thenperformed to compute precise GPS time (which may be accurate to within afew milliseconds in a current implementation. A Pattern Match Algorithmis disclosed in U.S. Pat. Nos. 5,812,087, 6,052,081, and 6,377,209 toNorman F. Krasner.

At 60, the GPS clock is then set to the computed, precise GPS time.Then, with the GPS time known, the location is fixed using anyappropriate procedures. It may be noted that after a location fix hasbeen performed, the GPS time is known to an accuracy of a fewnanoseconds. Therefore, after a location fix the GPS clock may be re-setwith this highly accurate time.

Extension to Allow Determination of Week Number

The previous discussion addresses the problem of establishing the timewithin a week, which is required to reference certain Assistance datatypically supplied by the server. However, this processing does notresolve the actual week number. The week number counts the number of GPSweeks that have occurred since the GPS clock was started. (The GPS clockwas started at 00:00 AM on Jan. 6, 1980). Certain data types may have afairly long life that extends beyond the current week; the SV Almanacbeing one example of this. Therefore occasionally a need arises toestablish a time estimate that also resolves the week number ambiguity.

In the IS-801 standard, the week number is transmitted by all SV's insubframe 1 (Bits 1:10 of the 3^(rd) word in subframe 1). Thisinformation may be embedded in the SA prediction data supplied by theIS-801 server, and therefore a suitable bit extraction code can be usedto isolate the week number field from the SA prediction data, and thenbe used to determine the week number.

It will be appreciated by those skilled in the art, in view of theseteachings, that alternative embodiments may be implemented withoutdeviating from the spirit or scope of the invention. This invention isto be limited only by the following claims, which include all suchembodiments and modifications when viewed in conjunction with the abovespecification and accompanying drawings.

What is claimed is:
 1. A method for setting coarse GPS time in a GPSreceiver comprising: a) requesting a sequence of predicted navigationbits; b) receiving the predicted navigation bits; c) saving a time ofreceipt of the navigation bits; d) locating a predicted time indicatorfield within the predicted navigation bits; e) determining a coarse timesetting in response to the located time indicator field; and f) settingcoarse GPS time within the GPS receiver responsive to the differencebetween the coarse time setting and the time of receipt.
 2. The methodof claim 1 further comprising performing a Pattern Match Algorithm toprovide precise GPS time.
 3. The method of claim 1 further comprising:a) determining an expected error in the coarse time setting; and b)setting coarse GPS time within the GPS receiver taking into account theexpected error in a GPS clock.
 4. The method of claim 1 wherein thepredicted navigation bits are received in a format that includes aplurality of frames, each frame organized into a plurality of subframes,each subframe having the time indicator field and including at least onesubframe of predicted navigation bits; the method further comprising:locating the predicted time indicator field within at least one subframeof the predicted navigation bits; and calculating the coarse GPS timefrom the predicted time indicator.
 5. The method of claim 1 wherein thepredicted navigation bits field is transmitted with a data length thatspecifies the length of the sequence of predicted navigation bits, and aReference Bit Number that designates the location of a predictednavigation bit within a frame of actual navigation bits, the methodfurther comprising: determining a location within a frame of actualnavigation bits, of a first bit within the sequence of predictednavigation bits based on the value of the Reference Bit Number and thedata length; locating the time indicator field within the predictednavigation bits based on the value of the Reference Bit Number; decodingthe located time indicator field to provide a predicted time indicator,determining the coarse GPS with respect to the time at which the firstbit of the sequence of predicted navigation bits was received; andcoincident with the first bit of the sequence of predicted navigationbits, setting coarse GPS time within the GPS receiver.
 6. The method ofclaim 1 wherein the MS and the base station are communicating using aGSM system.
 7. A mobile station for determining position utilizingperiodically transmitted navigation bits from a plurality of SVssynchronized with GPS time, the periodically transmitted navigation bitsincluding a time indicator field, the mobile station also communicatingwith one or more base stations and a position determining entity (PDE)comprising: a two-way communication system for communicating with thebase stations and the PDE; a position location system that includes aGPS clock; means for requesting an assistance message from the PDE, theassistance message including a sequence of predicted navigation bitssent from the base station approximately synchronized in time with GPStime; means for saving a time of receipt of the assistance message;means for locating the predicted time indicator field within thepredicted navigation bits; means, responsive to the located timeindicator field, for determining a predicted Time of Week; and means forsetting coarse GPS time within the GPS receiver responsive to thepredicted Time of Week and the time of receipt.
 8. The mobile station ofclaim 7 further comprising means, responsive to the coarse GPS time andthe predicted navigation bits, for performing a Pattern Match Algorithmto provide precise GPS time.
 9. The mobile station of claim 7 furthercomprising: means for determining an expected error in the Time of Week;and the means for setting coarse GPS time within the GPS receiverincludes means for setting the expected error in a GPS clock.
 10. Themobile station of claim 7 wherein the transmitted navigation bits have aformat including a plurality of frames, each frame organized into aplurality of subframes, each subframe having a time indicator field, andthe assistance message includes at least one subframe of predictednavigation bits, and further comprising: means for locating a predictedtime indicator field within a subframe of the predicted navigation bits;and means for calculating the Time of Week responsive to the predictedtime indicator.
 11. The mobile station of claim 10 wherein theassistance message includes a data length field that specifies thelength of the predicted navigation bits, and a Reference Bit Number thatdesignates a bit within a frame of the actual navigation bits, andfurther comprising: means, responsive to the Reference Bit Number fieldand the length field, for determining a First bit of the sequence ofpredicted navigation bits that corresponds to the position of the firstbit of the sequence within a frame of actual navigation bits; means,responsive to the position of the first bit of the sequence of predictednavigation bits, for locating a time indicator field within thepredicted navigation bits; means, responsive to the predicted timeindicator, for determining a Time of Week at the first bit of thesequence of predicted navigation bits; and means for setting coarse GPStime within the GPS receiver coincident with the first bit of thesequence of predicted navigation bits and responsive to the Time ofWeek.
 12. A method for synchronizing a GPS receiver with coarse GPS timein a mobile station (MS) communicating with a base station and aposition determining entity (PDE) using the IS-801 standard, the GPSreceiver configured to receive periodically transmitted navigation bitsfrom a plurality of SVs synchronized with GPS time, the transmittednavigation bits having a format including a plurality of frames, eachframe organized into a plurality of subframes, each subframe having aSUB-FRAME COUNT message, comprising: by the MS, requesting a SensitivityAssistance (SA) message from the PDE, the SA message including aPredicted Navigation Bits field that includes a sequence of predictednavigation bits including at least one subframe, a Data Record Sizefield that specifies the length of the Predicted Navigation Bits field,and a Reference Bit Number field that designates a bit within a frame ofthe actual navigation bits, thereby associating the predicted navigationbits with a group of navigation bits; responsive to the request from theMS, sending the SA message from the base station approximately in timewith GPS time; receiving the SA message in the MS, and saving a time ofreceipt of the SA message; responsive to the Reference Bit Number fieldand the Data Record Size field, determining a first bit of the sequenceof predicted navigation bits that corresponds to the position of thefirst bit of the sequence within a frame of actual navigation bits;responsive to the position of the first bit of the sequence of predictednavigation bits, locating the SUB-FRAME COUNT field within the predictednavigation bits; decoding the located SUB-FRAME COUNT field to provide apredicted SUB-FRAME COUNT value; responsive to the predicted SUB-FRAMECOUNT value, determining the Time of Week at the first bit of thesequence of predicted navigation bits; and coincident with the first bitof the sequence of predicted navigation bits, setting coarse GPS timewithin the GPS receiver responsive to the predicted SUB-FRAME COUNT andthe time of receipt.
 13. The method of claim 12 further comprisingdetermining an expected error in the Time of Week; and the step ofsetting coarse GPS time further includes setting the expected error. 14.The method of claim 13 wherein the predicted SUB-FRAME COUNT value isdefined with regard to a weekly time reference, and the step ofdetermining the Time of Week comprises computing a Bit of Weekcorresponding to the number of bits elapsed from the weekly timereference until the first bit of the sequence of predicted navigationbits, responsive to the predicted SUB-FRAME COUNT value and the positionof the first bit of the sequence of predicted navigation bits.
 15. Themethod of claim 14 wherein the step of computing a Bit of Week comprisesdetermining if the first bit of the sequence of predicted navigationbits is in the same subframe as the SUB-FRAME COUNT field, andresponsive thereto, adjusting the predicted SUB-FRAME COUNT value. 16.The method of claim 12 further comprising, responsive to the coarse GPStime and the predicted navigation bits, performing a Pattern MatchAlgorithm to provide precise GPS time.
 17. The method of claim 12wherein the MS and the base station are communicating using a GSMsystem.